1. Field of the Invention
The present invention is directed toward an improved transimpedance circuit for use in receiving a communications signal.
2. Description of the Related Art
Infrared receivers have become popular and useful devices for short range wireless communication of both analog and digital signals. Due to the high volume and widespread utilization of these receivers, competitive pressures have made it increasingly important that these receivers have optimum performance at low cost.
The Infrared Data Association (IrDA) organization has defined a new infrared protocol called IrDA Control (IrCtrl) that is intended for sending two-way short packet, control information for in room, very high volume consumer electronic appliance and video game applications. (See www.irda.org for further information regarding IrDA standards). Smarter two-way IrCtrl transceivers may eventually replace conventional very high volume one-way remote controls, allowing consolidation of several individual one-way remotes into a user-friendly two-way remote. In addition, the IrCtrl protocol is designed to allow multiple devices to work in a coordinated non-interfering way. This type of infrared wireless protocol is ideal for in room control applications, having advantages over short range RF protocols of local room level or point-and-beam addressing, an order of magnitude lower cost, and freedom from cumbersome RF regulatory issues.
The IrCtrl physical layer protocol uses a 1.5 Mhz carrier amplitude (on-off) modulated with a pulse width in multiples of 10 carrier cycles. It has a minimum range of 5 meters to allow operation across a typical room. This means that the receiver and photodiode combination needs to be 10 times more optically sensitive than the popular low speed (115.2 Kbps) IrDA SIR protocol. To achieve this signal sensitivity it must bandpass the signal to reject the increasingly common noisy AC infrared sources; such as, energy efficient high frequency ballast fluorescent lights, TVs, computer monitors, infrared wireless headphones, etc.
Due to variations in range from more than 0.2 to 5 meters and variations of 5 fold in transmit signal power, the receiver must be capable of handling signal level variations of 70 dB. This dynamic range is very large although less than the 100 dB dynamic range of the earlier IrDA SIR, MW FIR and VFIR physical layer protocols.
Finally, the receiver must be able to perform the above signal processing functions while handle variations in DC ambient current of from 0 to 10-100 uA. This current arises from ambient infrared light predominately from sunlight, radiant heaters, and incandescent light sources.
In order for an IrCtrl receiver to meet these stringent requirements four performance parameters are desirable for the receiver front end. These are 1) maximum sensitivity to minimize photodiode area and consequent cost, 2) good high frequency response despite significant photodiode capacitance, 3) large signal dynamic range, and 4) the ability to handle a large infrared ambient levels.
FIG. 1 illustrates an example of a photodiode 20 connected to a transimpedance amplifier 10. This is a well know method to convert the current signal output of a photodiode to a voltage signal output at output terminal DOUT. The transimpedance amplifier 10 provides a low impedance to the photodiode allowing good high frequency response. The transimpedance amplifier uses the principle of negative feedback to lower the impedance at the photodiode input while developing the photocurrent signal across feedback resistor 12. In FIG. 1, an embodiment is shown where the photodiode 20 is connected to the input of the transimpedance amplifier 10 though a DC blocking capacitor 14 that passes alternating current (AC) signals. The photodiode in this variation is also connected to a gyrator circuit, constructed of amplifiers 30 and 34 along with capacitor 32, that acts like an inductor blocking the AC signal currents but passing the DC photocurrent that arises from an infrared ambient signal.
In FIG. 1, if an operational amplifier with both wide bandwidth and low noise is used, good noise and frequency response can be achieved if care is taken that the gyrator circuit does not contribute excess noise. However, the circuit shown in FIG. 1 has no gain control. On strong signals, the transimpedance amplifier 10 will overload causing severe signal distortion or even complete loss of the signal received by photodiode 20. In addition, because this circuit has no automatic gain control (AGC) capability, even strong signals can be disrupted by weak interference in the signal periods between data pulses.
FIG. 2 illustrates an example of a photodiode receiver front end such as that disclosed in FIG. 11 of U.S. Pat. No. 5,864,591 to Holcombe. Although this front end circuit has a large gain control range that provides for the benefits of AGC, it trades off good noise performance for good frequency response. When input bias currents I1 and I2, provided by current sources 56 and 94, respectively, are set to a low level, which controls the emitter impedance of transistors 52 and 90, then the equivalent input current noise will be low thereby providing high sensitivity but with relatively poor frequency response. When the input bias currents are set sufficient for good frequency response, then the noise performance degrades and lowers the sensitivity. For low frequency, 115.2 Kbps SIR IrDA transceivers, this front end is adequate. The circuit shown in FIG. 2 is a classic differential structure. Providing a differential structures is a well known method for reducing spurious signals. In this example, the differential structure reduces the spurious signals arising from power supply noise, common bias current noise, and AGC gain control transients.
FIG. 3 illustrates another example of a photodiode receiver front end, similar to that disclosed by Jackson in U.S. Pat. No. 5,714,909. This structure, although providing good high frequency response and having good overload response, does not have the benefits of AGC.
Accordingly, the need remains for an improved infrared photodiode front end circuit having the performance characteristics sufficient for an IrCtrl infrared photodiode receiver.
Some of the problems with the prior art may be overcome through the following embodiments of the present invention.
An embodiment of a front end circuit, according to the present invention, for receiving and amplifying a signal includes a first output circuit node, a first input circuit node, first and second supply terminals, and a regulated supply terminal. The circuit also includes a first resistor coupled between the regulated supply terminal and the first output circuit node. A first transistor of the circuit has a control terminal and first and second current terminals, where the control terminal is coupled to the first output circuit node, the first current terminal is coupled to the first supply terminal, and the second current terminal is coupled to the first input circuit node. A second transistor of the circuit has a control terminal and first and second current terminals, where the control terminal of the second transistor is coupled to the first input circuit node, the first current terminal of the second transistor is coupled to the first output circuit node, and the second current terminal of the second transistor is coupled to the second supply terminal.
An embodiment of a method, according to the present invention, for receiving and amplifying a signal, involves providing a resistance between a first supply source and an output circuit node, providing a first transistor to sink current between the output circuit node and a second supply source, and receiving a signal at an input circuit node. The method further recites driving the first transistor using the received signal. The method also includes feeding back a signal from the output circuit node to the input circuit node through a second transistor.
An embodiment of a transimpedance amplifier circuit, according to the present invention, for receiving a signal includes a resistor coupled between a first supply source and an output circuit node of the amplifier circuit. The amplifier circuit also includes a first transistor configured to sink current from the output circuit node of the amplifier circuit to a second supply source, where a control terminal of the first transistor is coupled to an input circuit node of the amplifier circuit. The amplifier also utilizes a second transistor configured to source current from an AC grounded supply source to the input circuit node of the amplifier circuit, where a control terminal of the second transistor is coupled to the output circuit node.